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Wetlands are the largest natural source of methane (CH4); however, the contribution of subtropical wetlands to global CH4 budgets is still unclear due to difficulties in accurately quantifying CH4 emissions from these complex ecosystems. Both direct (water management strategies) and indirect (altered weather patterns associated with climate change) anthropogenic influences are also leading to greater uncertainties in our ability to determine changes in CH4 emissions from these ecosystems. This study compares CH4 fluxes from two freshwater marshes with different hydroperiods (short versus long) in the Florida Everglades to examine temporal patterns and biophysical drivers of CH4 fluxes. Both sites showed similar seasonal patterns across years with higher CH4 release during wet seasons versus dry seasons. The long hydroperiod site showed stronger seasonal patterns and overall, emitted more CH4 than the short hydroperiod site; however, no distinctive diurnal patterns were observed. We found that air temperature was a significant positive driver of CH4 fluxes for both sites regardless of season. In addition, gross ecosystem exchange was a significant negative predictor of CH4 emissions in the dry season at the long hydroperiod site. CH4 fluxes were impacted by water level and its changes over site and season, and time scales, which are influenced by rainfall and water management practices. Thus with increasing water distribution associated the Comprehensive Everglades Restoration Plan we expect increases in CH4 emissions, and when couple with increased with projected higher temperatures in the region, these increases may be enhanced, leading to greater radiative forcing. [ABSTRACT FROM AUTHOR]

How aquatic primary productivity influences the carbon (C) sequestering capacity of wetlands is uncertain. We evaluated the magnitude and variability in aquatic C dynamics and compared them to net ecosystem CO2 exchange (NEE) and ecosystem respiration (Reco) rates within calcareous freshwater wetlands in Everglades National Park. We continuously recorded 30-min measurements of dissolved oxygen (DO), water level, water temperature (Twater), and photosynthetically active radiation (PAR). These measurements were coupled with ecosystem CO2 fluxes over 5 years (2012–2016) in a long-hydroperiod peat-rich, freshwater marsh and a short-hydroperiod, freshwater marl prairie. Daily net aquatic primary productivity (NAPP) rates indicated both wetlands were generally net heterotrophic. Gross aquatic primary productivity (GAPP) ranged from 0 to − 6.3 g C m−2 day−1 and aquatic respiration (RAq) from 0 to 6.13 g C m−2 day−1. Nonlinear interactions between water level, Twater, and GAPP and RAq resulted in high variability in NAPP that contributed to NEE. Net aquatic primary productivity accounted for 4–5% of the deviance explained in NEE rates. With respect to the flux magnitude, daily NAPP was a greater proportion of daily NEE at the long-hydroperiod site (mean = 95%) compared to the short-hydroperiod site (mean = 64%). Although we have confirmed the significant contribution of NAPP to NEE in both long- and short-hydroperiod freshwater wetlands, the decoupling of the aquatic and ecosystem fluxes could largely depend on emergent vegetation, the carbonate cycle, and the lateral C flux. [ABSTRACT FROM AUTHOR]